Susumu Kodai, born in Kanagawa Prefecture, Japan, lost his family during an alien planet bombing attack on Earth. The subsequent death of his older brother Mamoru in space combat drove him toward military service. He enrolled in the Space Officer Training Academy, studying under Ryu Hijikata and Isami Enomoto, forming close bonds like the one with classmate Daisuke Shima. Initially fueled by intense anger and a desire for vengeance, he carried Mamoru's harmonica as a memento of his trauma and isolation.

Assigned first to the battleship Kirishima before joining the Yamato, he served as Tactical Officer under Captain Juzo Okita. His early command decisions often displayed recklessness, including abandoning his post during critical operations or initiating unsanctioned attacks that endangered the crew. This stemmed from deep-seated distrust of authority figures, particularly Okita, whom he initially blamed for his brother's death. Interstellar voyages exposed him to complex moral dilemmas involving alien civilizations and the sacrifice of comrades, gradually tempering his impulsiveness. His relationship with operations officer Yuki Mori evolved from professional tension to romantic commitment, culminating in an engagement.

During the confrontation with the Black Nebula Empire, he commanded the destroyer Yuunagi before returning to Yamato as acting captain. Tasked with neutralizing a planetary-destruction weapon on Dezarium, he faced critical choices when the enemy deployed a fabricated "future Earth" illusion to demoralize the crew. He maintained operational focus despite this psychological warfare. During the assault on Dezarium's core, he hesitated to fire the wave-motion gun upon learning his niece Sasha remained on the planet. After her death at the hands of Dezarium's leader Scaldart, he executed the attack, destroying Dezarium but triggering a chain reaction that necessitated an emergency warp escape. Following Captain Yamanami's mortal wounding in this battle, he assumed formal command of Yamato and completed the return voyage to Earth.

His psychological development reveals a transition from isolated rage to measured leadership. Early traits of pacifism and scientific curiosity, suppressed by wartime trauma, reemerged through experiences emphasizing cooperation over conflict. His persistent sense of loneliness lessened through bonds formed with the crew, particularly Mori. Command responsibilities compelled him to prioritize collective survival over personal vendettas, evident in his acceptance of ceasefires with former enemies and his focus on protecting civilian lives during later campaigns. Post-mission reflections often involved questioning the moral compromises of Earth's rebuilt society, indicating enduring idealism shaped by loss and redemption.